It is known that extremely precise control of the distance between backplate and diaphragm is very important in an electret microphone. Desirably, variation in the distance from unit to unit should be less than two microns, the distance itself typically being in the range of from 25 to 40 microns.
Techniques heretofore proposed for controlling this distance, or separation, or spacing, have included protrusions (Schmitt U.S. Pat. No. 3,772,133, dated Nov. 13, 1973), a screen (Smulders U.S. Pat. No. 4,160,881, dated July 10, 1979), and directly securing a spacer to a Teflon electret layer on a backplate by heating, the spacer itself being spaced from the backplate by the thickness of the Teflon layer.
In hearing aid microphones the sound entry port is usually on a narrow edge of the microphone rather than the front face, and the diaphragm is supported on a ring or bonded to the backplate. In order to provide an unobstructed path from the entry port to the diaphragm, so called acoustic terminators (e.g, U.S. Pat. No. 4,331,840) or deformations in the housing (e.g., U.S. Pat. No. 4,160,881) have been used.
The electret microphones used in hearing aids have used hybrid rather than monolithic preamplifiers, because only the former were able to provide sufficiently low noise, low current drain, and low operating voltage (pinch-off voltage 0.3 to 0.7 volt). Recently, monolithic JFET preamplifiers have been proposed for hearing aids.